Conventionally, the frequency spectrum of the receiver of the communicating system has a large scale variation and the precision wireless signal received by the receiver is affected by not only the amplitude of the wanted signal but also the interference signal (or termed “jammer”) which is inherent in the adjacent frequency bands. Therefore, when the receiver detects the minimum and maximum power of the waned signal, and the interference signal is inherent in the adjacent frequency bands, the receiver has to further process the wireless signal.
As shown in FIG. 1, it is a schematic block diagram of conventional receiver 100 with an automatic gain control. The receiver 100 includes an antenna 101, a low noise amplifier (LNA) 102, a frequency converter 103, a channel selection filter (CSF) 104 and a programmable gain amplifier (PGA) 105.
The receiver 100 receives the ultra-high signal by the antenna 101 and the low noise amplifier (LNA) 102 amplifies the received signal. The frequency converter 103 down-converts the frequency of the received signal. As shown in FIG. 2, it is a schematic view of a circuit diagram of a channel selection filter 104 of biquad operational amplifier. The channel selection filter 104 filters away the interference signal of the received signal. As shown in FIG. 3, it is a schematic view of a circuit diagram of a programmable gain amplifier 105. The programmable gain amplifier 105 composed of amplifiers amplifies the received signal and outputs a signal at a predetermined amplitude.
The transfer function of the channel selection filter (CSF) 104 is represented as formula 1 (E1), and the transfer function of the programmable gain amplifier (PGA) 105 is represented as formula 2 (E2):
                                          V            o                                V            i                          =                                                            S                2                            ⁢                              C                2                            ⁢                              C                3                                      +                          S              ⁢                                                          ⁢                                                                    C                    1                                    -                                      C                    x                                                                    R                  3                                                      +                          1                                                R                  1                                ⁢                                  R                  3                                                                                                        S                2                            ⁢                              C                2                            ⁢                              C                4                                      +                          S              ⁢                                                          ⁢                                                C                  2                                                  R                  4                                                      +                          1                                                R                  2                                ⁢                                  R                  3                                                                                        (        E1        )                                Gain        =                  -                                    R              302                                      R              301                                                          (        E2        )            
In addition, the receiver 100 further includes a first peak detector 1 (PD1) 107, a second peak detector (PD2) 109, a first automatic gain control (AGC1) 108, a second automatic gain control (AGC2) 110. The first peak detector (PD1) 107 is used to detect the signal before being inputted into the channel selection filter (CSF) 104, and the second peak detector (PD2) 109 is used to detect the signal after being outputted from the channel selection filter (CSF) 104. The first automatic gain control (AGC1) 108 adjusts the gain of the low noise amplifier 102 and frequency converter 103 according to the first detecting value of the first peak detector (PD1) 107. The second automatic gain control (AGC2) 110 adjusts the gain of the channel selection filter (CSF) 104 and programmable gain amplifier 105 according to the second detecting value of the second peak detector (PD2) 109.
Therefore, the first peak detector (PD1) 107 in front of the channel selection filter (CSF) 104 detects the total amplitude of the inputted signal including the wanted signal and interference signal which is inherent in the inputted signal. The first automatic gain control (AGC1) 108 adjusts the gain of the low noise amplifier 102 and frequency converter 103 according to the total amplitude of the inputted signal, so that the inputted signal before the channel selection filter (CSF) 104 maintains the amplitude constant. The receiver 100 prevents the signal from distortion due to the side effect of the interference signal.
Moreover, the second peak detector (PD2) 109 is after the channel selection filter (CSF) filters 104 that reduces the interference signal. The second peak detector (PD2) 109 detects the amplitude of the signal which is composed of wanted signal, and the second automatic gain control (AGC2) 110 then adjusts the gain of the channel selection filter (CSF) 104 and programmable gain amplifier 105 according to the amplitude for maintaining the amplitude constant.
As shown in FIG. 4, it is a schematic view of the characteristics profile of out-band 3rd input intercept point (IIP3) of an operational amplifier, corresponding to the interference signal. With the increment of the gain-bandwidth (GBW), the current associated with the gain-bandwidth (GBW) is increased positively. When the value of the 3rd input intercept point circuit (IIP3) is higher, the distortion of the inputted signal is decreased. For the purpose of lower distortion, it is necessary to provide higher current for the optimal 3rd input intercept point (IIP3). However, such a situation considerably increases the power consumption of the receiver 100.
Conventionally, when the channel selection filter (CSF) 104 and programmable gain amplifier 105 are utilized in the receiver, only the current satisfying the noise at the optimal point is required. Unfortunately, as shown in FIG. 5, for achieving better immunity from interference signal, it is necessary to provide higher current than the current of the noise at the optimal point, i.e. supply the higher current to the low noise amplifier 102 and frequency converter 103, so that an optimal point that the noise is minimum is achieved, and the optimal 3rd input intercept point (IIP3) is maximum.
The frequency spectrum of the receiver of the communicating system has a large scale variation, and the interference signal is sometimes inherent in the adjacent frequency bands of the wanted signal.
Since the interference signal has ill effects on the wanted signal, the channel selection filter is used to filter the interference signal and reduce the amplitude of the interference signal. However, when the channel selection filter (CSF) 104 and programmable gain amplifier 105 have to be operated precisely in the worst case, i.e. the wanted signal is low, and the interference signal is high, it is required to exert high current on the channel selection filter (CSF) 104 and programmable gain amplifier 105. Such that the signal distortion is minimum, and the optimal 3rd input intercept point (IIP3) is maintained.
It is necessary to provide high current not only in the worst case, as mentioned above, but also when the interference signal is not inherent in the wanted signal. Therefore, high current is provided to the receiver for maintaining the optimal 3rd input intercept point (IIP3), and excessive current is thus supplied to the receiver, thereby resulting in undue power consumption.
Furthermore, when no interference signal exists, and the amplitude of the wanted signal is higher and even though the characteristics profile of the noise is bad and the distortion of the signal is small, the receiver still consumes large power. Consequentially, there is a need to develop a novel receiver solve the aforementioned problem of power consumption.